Acute alcohol tolerance is intrinsic to the BKCa protein, but is modulated by the lipid environment

Ethanol tolerance, in which exposure leads to reduced sensitivity, is an important component of alcohol abuse and addiction. The molecular mechanisms underlying this process remain poorly understood. The BKCa channel plays a central role in the behavioral response to ethanol in Caenorhabditis elegans (Davies, A. G., Pierce-Shimomura, J. T., Kim, H., VanHoven, M. K., Thiele, T. R., Bonci, A., Bargmann, C. I., and McIntire, S. L. (2003) Cell 115, 655-666) and Drosophila (Cowmeadow, R. B., Krishnan, H. R., and Atkinson, N. S. (2005) Alcohol. Clin. Exp. Res. 29, 1777-1786) . In neurons, ethanol tolerance in BKCa channels has two components: a reduced number of membrane channels and decreased potentiation of the remaining channels (Pietrzykowski, A. Z., Martin, G. E., Puig, S. I., Knott, T. K., Lemos, J. R., and Treistman, S. N. (2004) J. Neurosci. 24, 8322-8332) . Here, heterologous expression coupled with planar bilayer techniques examines two additional aspects of tolerance in human BKCa channels. 1) Is acute tolerance observed in a single channel protein complex within a lipid environment reduced to only two lipids? 2) Does lipid bilayer composition affect the appearance of acute tolerance? We found that tolerance was observable in BKCa channels in membrane patches pulled from HEK cells and when they are placed into reconstituted 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylserine membranes. Furthermore, altering bilayer thickness by incorporating the channel into lipid mixtures of 1,2-dioleoyl-3-phosphatidylethanolamine with phosphatidylcholines of increasing chain length, or with sphingomyelin, strongly affected the sensitivity of the channel, as well as the time course of the acute response. Ethanol sensitivity changed from a strong potentiation in thin bilayers to inhibition in thick sphingomyelin/1,2-dioleoyl-3-phosphatidylethanolamine bilayers. Thus, tolerance can be an intrinsic property of the channel protein-lipid complex, and bilayer thickness plays an important role in shaping the pattern of response to ethanol. As a consequence of these findings the protein-lipid complex should be treated as a unit when studying ethanol action.     

Gating and Conductance Changes in BK Ca Channels in Bilayers Are Reciprocal

The energy associated with a mismatch between the hydrocarbon portions of a lipid bilayer and the hydrophobic regions of a transmembrane protein requires that one or both components deform in an attempt to minimize the energy difference. Transmembrane potassium channel subunits are composed of different structural motifs, each responsible for ion-selectivity, conductance and gating capabilities. Each has an inherent degree of flexibility commensurate with its amino acid composition. It is not clear, however, how each structural motif will respond to a fixed amount of distortion applied to the whole structure. We examined the single-channel conductance (G_c) and gating (open probability, P _o) of single BK_Ca channels (hslo α-subunits) inserted into planar lipid bilayers containing 1,2-dioleoyl-3-phosphatidylethanolamine (DOPE) or DOPE with either 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) or sphingomyelin (SPM) and 1-palmitoyl-2-oleoyl-3-phosphatidylethanolamine (POPE) with SPM. These latter three binary mixtures formed stable membranes with different distributions of thickness domains as determined by atomic force microscopy. Channels placed in each composition should be exposed to different amounts of distortion. BK_Ca channels forced into the DOPE/SPM bilayer containing lipid domains with two different thicknesses showed two distinct levels of G_c and P_o. The alterations in G_c and P_o were reciprocal. A larger conductance was accompanied by a smaller value for gating and vice versa. Channels forced into the POPE/SPM bilayer containing lipid domains with different thicknesses showed more than two distinct levels of G_c and P_o. Channels placed in a uniform bilayer (DOPE/DOPC) showed a uniform distribution of conductance and activation. We conclude that both the inner and outer domains of the channel where these two channel functions are localized respond to deformation and that a fixed amount of distortion results in reciprocal changes in protein function.     

N-acyl phosphatidylethanolamines affect the lateral distribution of cholesterol in membranes

N-Acyl phosphatidylethanolamines are negatively charged phospholipids, which are naturally occurring albeit at low abundance. In this study, we have examined how the amide-linked acyl chain affected the membrane behavior of the N-acyl-1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine (N-acyl-POPE) or N-acyl-dipalmitoyl-sn-glycero-3-phosphatidylethanolamine (N-acyl-DPPE), and how the molecules interacted with cholesterol. The gel-->liquid crystalline transition temperature of sonicated N-acyl phosphatidylethanolamine vesicles in water correlated positively with the number of palmitic acyl chains in the molecules. Based on diphenylhexatriene steady state anisotropy measurements, the presence of 33 mol% cholesterol in the membranes removed the phase transition from N-oleoyl-POPE bilayers, but failed to completely remove it from N-palmitoyl-DPPE and N-palmitoyl-POPE bilayers, suggesting rather weak interaction of cholesterol with the N-saturated NAPEs. The rate of cholesterol desorption from mixed monolayers containing N-palmitoyl-DPPE and cholesterol (1:1 molar ratio) was much higher compared to cholesterol/DPPE binary monolayers, suggesting a weak cholesterol interaction with N-palmitoyl-DPPE also in monolayers. In bilayer membranes, both N-palmitoyl-POPE and N-palmitoyl-DPPE failed to form sterol-rich domains, and in fact appeared to displace sterol from sterol/N-palmitoyl-sphingomyelin domains. The present data provide new information about the effects of saturated NAPEs on the lateral distribution of cholesterol in NAPE-containing membranes. These findings may be of relevance to neural cells which accumulate NAPEs during stress and cell injury.     

Lipids Modulate the Increase of BK Channel Calcium Sensitivity by the β1 Subunit

Co-expression of the auxiliary β1 subunit with the pore forming α subunit of BK dramatically alters apparent calcium sensitivity. Investigation of the mechanism underlying the increase in calcium sensitivity of BK in smooth muscle has concentrated on the energetic effect of β1′s interaction with α. We take a novel approach, exploring whether β1 modification of calcium sensitivity reflects altered interaction between the channel protein and surrounding lipids. We reconstituted hSlo BK α and BK α+β1 channels into two sets of bilayers. One set contained POPE with POPS, POPG, POPA and POPC, where the length of acyl chains is constant, but surface charge differs. The second set is a series of neutral bilayers formed from DOPE with phosphatidylcholines (PCs) of varying acyl chain lengths: C (14∶1), C (18∶1), C (22∶1) and C (24∶1), and with brain sphingomyelin (SPM), in which surface charge is constant, but bilayer thickness varies. The increase in calcium sensitivity caused by the β1 subunit was preserved in negatively charged lipid bilayers but not in neutral bilayers, indicating that modification of apparent Ca²⁺ sensitivity by β1 is modulated by membrane lipids, requiring negatively charged lipids in the membrane. Moreover, the presence of β1 reduces BK activity in thin bilayers of PC 14∶1 and thick bilayers containing SPM, but has no significant effect on activity of BK in PC 18∶1, PC 22∶1 and PC 24∶1 bilayers. These data suggest that auxiliary β1 subunits fine-tune channel gating not only through direct subunit-subunit interactions but also by modulating lipid-protein interactions.     

Bilayer thickness modulates the conductance of the BK channel in model membranes

The conductance of the BK channel was evaluated in reconstituted bilayers made of POPE/POPS (3.3:1), or POPE/POPS with an added 20% of either SPM (3.3:1:1), CER (3.3:1:1), or CHL (3.3:1:1). The presence of SPM, which is known to increase bilayer thickness, significantly reduced the conductance of the BK channel. To directly test the role of membrane thickness, the conductance of the BK channel was measured in bilayers formed from PCs with acyl chains of increasing length (C14:1-C24:1), all in the absence of SPM. Slope conductance was maximal at a chain length of (C18:1) and much reduced for both thinner (C14:1) and thicker (C24:1) bilayers, indicating that membrane thickness alone can modify slope conductance. Further, in a simplified binary mixture of DOPE/SPM that forms a confined, phase-separated bilayer, the measured conductance of BK channels shows a clear bimodal distribution. In contrast, the addition of CER, which has an acyl chain structure similar to SPM but without its bulky polar head group to POPE/POPS, was without effect, as was the addition of CHL. The surface structure of membranes made from these same lipid mixtures was examined with AFM. Incorporation of both SPM and CER resulted in the formation of microdomains in POPE/POPS monolayers, but only SPM promoted a substantial increase in the amount of the high phase observed for the corresponding bilayers. The addition of CHL to POPE/POPS eliminated the phase separation observed in the POPE/POPS bilayer. The decrease in channel conductance observed with the incorporation of SPM into POPE/POPS membranes was, therefore, attributed to larger SPM-rich domains that appear thicker than the neighboring bilayer.     

Limiting an antimicrobial peptide to the lipid-water interface enhances its bacterial membrane selectivity: a case study of MSI-367

In a minimalist design approach, a synthetic peptide MSI-367 [(KFAKKFA)(3)-NH(2)] was designed and synthesized with the objective of generating cell-selective nonlytic peptides, which have a significant bearing on cell targeting. The peptide exhibited potent activity against both bacteria and fungi, but no toxicity to human cells at micromolar concentrations. Bacterial versus human cell membrane selectivity of the peptide was determined via membrane permeabilization assays. Circular dichroism investigations revealed the intrinsic helix propensity of the peptide, β-turn structure in aqueous buffer and extended and turn conformations upon binding to lipid vesicles. Differential scanning calorimetry experiments with 1,2-dipalmitoleoyl-sn-glycero-3-phosphatidylethanolamine bilayers indicated the induction of positive curvature strain and repression of the fluid lamellar to inverted hexagonal phase transition by MSI-367. Results of isothermal titration calorimetry (ITC) experiments suggested the possibility of formation of specific lipid-peptide complexes leading to aggregation. (2)H nuclear magnetic resonance (NMR) of deuterated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) multilamellar vesicles confirmed the limited effect of the membrane-embedded peptide at the lipid-water interface. (31)P NMR data indicated changes in the lipid headgroup orientation of POPC, 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylglycerol, and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine lipid bilayers upon peptide binding. Membrane-embedded and membrane-inserted states of the peptide were observed via sum frequency generation vibrational spectroscopy. Circular dichroism, ITC, and (31)P NMR data for Escherichia coli lipids agree with the hypothesis that strong electrostatic lipid-peptide interactions embrace the peptide at the lipid-water interface and provide the basis for bacterial cell selectivity.     

Kinetics and thermodynamics of annexin A1 binding to solid-supported membranes: a QCM study

By means of the quartz crystal microbalance (QCM) technique, the interaction of annexin A1 with lipid membranes was quantified using solid-supported bilayers immobilized on gold electrodes deposited on 5 MHz quartz plates. Solid-supported lipid bilayers were composed of a first octanethiol monolayer chemisorbed on gold and a physisorbed phospholipid monolayer obtained from vesicle fusion. This experimental setup enabled us to determine for the first time rate constants and affinity constants of annexin A1 binding to phosphatidylserine-containing layers as a function of the calcium ion concentration in solution and the cholesterol content within the outer leaflet of the solid-supported bilayer. The results reveal that a decrease in Ca(2+) concentration from 1 mM to 100 microM significantly increases the rate of annexin A1 binding to the membrane independent of the cholesterol content. However, the presence of cholesterol in the membrane altered the affinity constants considerably. While the association constant decreases with decreasing Ca(2+) concentration in the case of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC)/1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) membranes lacking cholesterol, it remains high in the presence of cholesterol.     

N-cholesteryl sphingomyelin-A synthetic sphingolipid with unique membrane properties

A sphingomyelin chimera in which the amide-linked acyl chain was replaced with cholesterol carbamate was prepared and its properties examined. The sphingomyelin/cholesterol chimera (N-cholesterol-D-erythro-sphingomyelin) was able to form unilamellar vesicles of defined size when extruded through 200nm pore size membranes. These N-cholesteryl sphingomyelin bilayers were resistant to solubilization by Triton X-100. When N-cholesteryl sphingomyelin was added to N-palmitoyl sphingomyelin (N-palmitoyl-d-erythro-sphingomyelin) bilayers, it increased acyl chain order as determined by 1,6-diphenyl-1,3,5-hexatriene fluorescence anisotropy. N-cholesteryl sphingomyelin was, however, not as good an inducer of membrane order compared to cholesterol on a molar basis. Differential scanning calorimetry studies further showed that the miscibility of N-cholesteryl sphingomyelin with N-palmitoyl-d-erythro-sphingomyelin bilayers was non-ideal, and the effect of N-cholesteryl sphingomyelin on the N-palmitoyl-d-erythro-sphingomyelin gel-fluid transition enthalpy differed from that seen with cholesterol. Together with N-palmitoyl-d-erythro-sphingomyelin, the N-cholesteryl sphingomyelin chimera was able to form sterol-enriched ordered domains in a fluid 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer. N-cholesteryl sphingomyelin in the absence of N-palmitoyl-d-erythro-sphingomyelin was unable to form such sterol-enriched ordered domains in a 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayer. However, N-cholesteryl sphingomyelin markedly increased the affinity of cholestatrienol for N-cholesteryl sphingomyelin containing 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine bilayers, suggesting that N-cholesteryl sphingomyelin was able to somehow stabilize sterol interaction in fluid bilayers. Based on our results, we conclude that N-cholesteryl sphingomyelin behaved more like a cholesterol than a sphingolipid in fluid bilayer membranes. Because N-cholesteryl sphingomyelin increased bilayer order, conferred resistance against detergent solubilization, and is not degradable by phospholipases A(2), it could constitute a good lipocomplex matrix for drug delivery vehicles.     

Comparative study of stability and transport of molecules through cyclic peptide nanotube and aquaporin: a molecular dynamics simulation approach

Abstract

The structural stability and transport properties of the cyclic peptide nanotube (CPN) 8?×?[Cys–Gly–Met–Gly]₂ in different phospholipid bilayers such as POPA (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidic acid), POPE (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine), POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine), POPG (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol) and POPS (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine) with water have been investigated using molecular dynamics (MD) simulation. The hydrogen bonds and non-bonded interaction energies were calculated to study the stability in different bilayers. One µs MD simulation in POPA lipid membrane reveals the stability of the cyclic peptide nanotube, and the simulations at various temperatures manifest the higher stability of 8?×?[Cys–Gly–Met–Gly]₂. We demonstrated that the presence of sulphur-containing amino acids in CPN enhances the stability through disulphide bonds between the adjacent rings. Further, the water permeation coefficient of the CPN is calculated and compared with human aquaporin-2 (AQP2) channel protein. It is found that the coefficients are highly comparable to the AQP2 channel though the mechanism of water transport is not similar to AQP 2; the flow of water in the CPN is taking place as a two-line 1–2–1–2 file fashion. In addition to that, the transport behavior of Na⁺ and K⁺ ions, single water molecule, urea and anti-cancer drug fluorouracil were investigated using pulling simulation and potential of mean force calculation. The above transport behavior shows that Na⁺ is trapped in CPN for a longer time than other molecules. Also, the interactions of the ions and molecules in Cα and mid-Cα plane were studied to understand the transport behavior of the CPN. Abbreviations AQP2 Aquaporin-2

CPN Cyclic peptide nanotube

MD Molecular dynamics

POPA 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphatidic acid

POPE 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine

POPG 1-Palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol

POPS 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine

Communicated by Ramaswamy H. Sarma     

Binding of bovine seminal plasma protein BSP-A1/-A2 to model membranes: lipid specificity and effect of the temperature

Bovine seminal plasma (BSP) contains a family of phospholipid-binding proteins. The affinity of the protein BSP-A1/-A2 for lipid membranes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), and POPC containing 30% (mol/mol) 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE) or cholesterol, has been investigated by the isothermal titration calorimetry (ITC). This study confirms the association of these proteins to lipid bilayers, and provides a direct characterization of this exothermic process, at 37 degrees C. The measurements indicate that the protein affinity for lipid bilayers is modulated by the lipid composition, the lipid/protein ratio, and the temperature. The saturation lipid/protein ratio was increased in the presence of cholesterol and, to a lesser extent, of phosphatidylethanolamine, suggesting that it is modulated by the lipid acyl chain order. For all the investigated systems, the binding of BSP-A1/-A2 could not be modeled using a simple partitioning of the proteins between the aqueous and lipid phases. The existence of "binding sites", and lipid phase separations is discussed. The decrease of temperature, from 37 to 10 degrees C, converts the exothermic association of the proteins to the POPC bilayers to an endothermic process. A complementary 1-D and 2-D infrared spectroscopy study excludes the thermal denaturation of BSP-A1/-A2 as a contributor in the temperature dependence of the protein affinity for lipid bilayers. The reported findings suggest that changes in the affinity of BSP-A1/-A2 for lipid bilayers could be involved in modulating the association of these proteins to sperm membranes as a function of space and time; this would consequently modulate the extent of lipid extraction, including cholesterol, at a given place and given time.     

Thermodynamics of insertion and self-association of a transmembrane helix: a lipophobic interaction by phosphatidylethanolamine

Thermodynamic parameters for the insertion and self-association of transmembrane helices are important for understanding the folding of helical membrane proteins. The lipid composition of bilayers would significantly affect these fundamental processes, although how is not well understood. Experimental systems using model transmembrane helices and lipid bilayers are useful for measuring and interpreting thermodynamic parameters (ΔG, ΔH, ΔS, and ΔC(p)) for the processes. In this study, the effect of the charge, phase, acyl chain unsaturation, and lateral pressure profile of bilayers on the membrane partitioning of the transmembrane helix (AALALAA)(3) was examined. Furthermore, the effect of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine (POPE) on the thermodynamics for insertion and self-association of the helix in host membranes composed of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylcholine (POPC) was investigated in detail. Interbilayer transfer of the helix monomer from POPC to POPC/POPE (1/1) bilayers was unfavorable (ΔG = +4.5 ± 2.9 kJ mol(-1) at 35 °C) due to an increase in enthalpy (ΔH = +31.1 ± 2.1 kJ mol(-1)). On the other hand, antiparallel dimerization of the helices in POPC/POPE (1/1) bilayers was enhanced compared with that in POPC bilayers (ΔΔG = -4.9 ± 0.2 kJ mol(-1) at 35 °C) due to a decrease in enthalpy (ΔΔH = -33.2 ± 1.5 kJ mol(-1)). A greater thickness of POPC/POPE bilayers only partially explained the observed effects. The residual effects could be related to changes in other physical properties such as higher lateral pressure in the hydrocarbon core in the PE-containing membrane. The origin of the enthalpy-driven "lipophobic" force that modulates the insertion and association of transmembrane helices will be discussed.     

The second transmembrane domain of the large conductance, voltage- and calcium-gated potassium channel beta(1) subunit is a lithocholate sensor

Bile acids and other steroids modify large conductance, calcium- and voltage-gated potassium (BK) channel activity contributing to non-genomic modulation of myogenic tone. Accessory BK beta(1) subunits are necessary for lithocholate (LC) to activate BK channels and vasodilate. The protein regions that sense steroid action, however, remain unknown. Using recombinant channels in 1-palmitoyl-2-oleoyl-phosphatidylethanolamine/1-palmitoyl-2-oleoyl-phosphatidylserine bilayers we now demonstrate that complex proteolipid domains and cytoarchitecture are unnecessary for beta(1) to mediate LC action; beta(1) and a simple phospholipid microenvironment suffice. Since beta(1) senses LC but beta(4) does not, we made chimeras swapping regions between these subunits and, following channel heterologous expression, demonstrate that beta(1) TM2 is a bile acid-recognizing sensor.     

Fluid-phase chain unsaturation controlling domain microstructure and phase in ternary lipid bilayers containing GalCer and cholesterol

We report the microstructure and phase behavior of three ternary mixtures each containing a long-chain saturated glycosphingolipid, galactosylceramide (GalCer), and cholesterol at room temperature. The unsaturation level of the fluid-phase component was varied by lipid choice, i.e., saturated 1,2-dilauroyl-sn-glycero-3-phosphocholine (DLPC), singly unsaturated 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), or doubly unsaturated 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). GalCer was used because of its biological significance, for example, as a ligand in the sexual transmission of HIV and stimulator of natural killer T-cells. Supported lipid bilayers of the ternary mixtures were imaged by atomic force microscopy and GalCer-rich domains were characterized by area/perimeter ratios (A/P). GalCer domain phase transitions from solid (S) to liquid (L) phase were verified by domain behavior in giant unilamellar vesicles, which displayed two-dimensional microstructure similar to that of supported lipid bilayers. As cholesterol concentration was increased, we observed approximately 2.5, approximately 10, and approximately 20-fold decreases in GalCer domain A/P for bilayers in L-S phase coexistence containing DOPC, POPC, and DLPC, respectively. The transition to L-L phase coexistence occurred at approximately 10 mol % cholesterol for bilayers containing DOPC or POPC and was accompanied by maintenance of a constant A/P. L-L phase coexistence did not occur for bilayers containing DLPC. We systematically relate our results to the impact of chain unsaturation on the interaction of the fluid-phase lipid and cholesterol. Physiologically, these observations may give insight into the interplay of fatty acid chain unsaturation, sterol concentration, and lipid hydrophobic mismatch in membrane phenomena.     

Importance of phosphatidylethanolamine for the interaction of apocytochrome c with model membranes containing phosphatidylserine

The effect of phosphatidylethanolamine (PE) on the binding of apocytochrome c to model membranes was examined. When 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC) of the standard vesicles composed of 80% of this lipid and 20% of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoserine (POPS) was gradually replaced with upward of 50% of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoethanolamine (POPE), the binding increased appreciably. Ca(2+), causing the phase separation of PS, also brought about increased binding of apocytochrome c in the PC/PS system, underlining the importance of PS properties in membranes for the protein binding. The resonance energy transfer between Trp-59 in apocytochrome c and pyrene-PS incorporated into bilayers showed that the replacement of PC with PE increased the extent of apocytochrome c penetration into membranes by a PE concentration-dependent manner. However, in the absence of PS, PE had no apparent effect on these functions of apocytochrome c, suggesting that PE-induced change(s) of acidic membrane properties is important to the association of apocytochrome c with vesicles. From the observations that the excimer to monomer fluorescence ratio of pyrene-PS increased and the fluorescence of NBD-PS was quenched with increasing concentration of PE, it was deduced that PE caused PS-enriched domains in PC/PE/PS membranes. The colocalization of pyrene-PS with BODIPY-PS by PE further supported the possibility. We suggest that PE-induced formation of PS-enriched domains acts as binding sites for apocytochrome c in membranes.     

Interaction of phosphatidylserine synthase from E. coli with lipid bilayers: coupled plasmon-waveguide resonance spectroscopy studies

The interaction of phosphatidylserine (PS) synthase from Escherichia coli with lipid membranes was studied with a recently developed variant of the surface plasmon resonance technique, referred to as coupled plasmon-waveguide resonance spectroscopy. The features of the new technique are increased sensitivity and spectral resolution, and a unique ability to directly measure the structural anisotropy of lipid and proteolipid films. Solid-supported lipid bilayers with the following compositions were used: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC); POPC-1-palmitoyl-2-oleoyl-sn-glycero-3-phosphate (POPA) (80:20, mol/mol); POPC-POPA (60:40, mol/mol); and POPC-1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(1-glycerol)] (POPG) (75:25, mol/mol). Addition of either POPA or POPG to a POPC bilayer causes a considerable increase of both the bilayer thickness and its optical anisotropy. PS synthase exhibits a biphasic interaction with the bilayers. The first phase, occurring at low protein concentrations, involves both electrostatic and hydrophobic interactions, although it is dominated by the latter, and the enzyme causes a local decrease of the ordering of the lipid molecules. The second phase, occurring at high protein concentrations, is predominantly controlled by electrostatic interactions, and results in a cooperative binding of the enzyme to the membrane surface. Addition of the anionic lipids to a POPC bilayer causes a 5- to 15-fold decrease in the protein concentration at which the first binding phase occurs. The results reported herein lend experimental support to a previously suggested mechanism for the regulation of the polar head group composition in E. coli membranes.     

Nisin promotes the formation of non-lamellar inverted phases in unsaturated phosphatidylethanolamines

Nisin, a peptide used as a food preservative, is shown, by 31P-nuclear magnetic resonance and infrared spectroscopy, to perturb the structure of membranes formed of unsaturated phosphatidylethanolamine (PE) and to induce the formation of inverted non-lamellar phases. In the case of dioleoyl-sn-glycero-3-phosphatidylethanolamine (DOPE), nisin promotes the formation of inverted hexagonal phase. Similarly, the peptide induces the formation of an isotropic phase, most likely a cubic phase, with 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphatidylethanolamine (POPE). It is proposed that the insertion of the peptide in the bilayer shifts the amphiphilic balance by increasing the hydrophobic contribution and is at the origin of the changes in the polymorphic propensities of PE. This is supported by the fact that the presence of cholesterol in the PE bilayer inhibits the power of nisin to perturb the membrane structure, most likely because the peptide insertion is difficult in the fluid ordered phase. This finding provides insight into possible antibacterial mechanisms of nisin.     

Isothermal titration calorimetry studies of the binding of a rationally designed analogue of the antimicrobial peptide gramicidin s to phospholipid bilayer membranes

The binding of the positively charged antimicrobial peptide cyclo[VKLdKVdYPLKVKLdYP] (GS14dK4) to various lipid bilayer model membranes was investigated using isothermal titration calorimetry. GS14dK4 is a diastereomeric lysine ring-size analogue of the naturally occurring antimicrobial peptide gramicidin S which exhibits enhanced antimicrobial and markedly reduced hemolytic activities compared with GS itself. Large unilamellar vesicles composed of various zwitterionic (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphorylcholine [POPC]) and anionic phospholipids {1-palmitoyl-2-oleoyl-sn-glycero-3-[phospho-rac-(glycerol)] [POPG] and 1-palmitoyl-2-oleoyl-sn-glycero-3-[phosphoserine] [POPS]}, with or without cholesterol, were used as model membrane systems. Dynamic light scattering results indicate the absence of any peptide-induced major alteration in vesicle size or vesicle fusion under our experimental conditions. The binding of GS14dK4 is significantly influenced by the surface charge density of the phospholipid bilayer and by the presence of cholesterol. Specifically, a significant reduction in the degree of binding occurs when three-fourths of the anionic lipid molecules are replaced with zwitterionic POPC molecules. No measurable binding occurs to cholesterol-containing zwitterionic vesicles, and a dramatic drop in binding is observed in the cholesterol-containing anionic POPG and POPS membranes, indicating that the presence of cholesterol markedly reduces the affinity of this peptide for phospholipid bilayers. The binding isotherms can be described quantitatively by a one-site binding model. The measured endothermic binding enthalpy (DeltaH) varies dramatically (+6.3 to +26.5 kcal/mol) and appears to be inversely related to the order of the phospholipid bilayer system. However, the negative free energy (DeltaG) of binding remains relatively constant (-8.5 to -11.5 kcal/mol) for all lipid membranes examined. The relatively small variation of negative free energy of peptide binding together with a pronounced variation of positive enthalpy produces an equally strong variation of TDeltaS (+16.2 to +35.0 kcal/mol), indicating that GS14dK4 binding to phospholipids bilayers is primarily entropy driven.     

A charge-modified general amber force field for phospholipids: improved structural properties in the tensionless ensemble

Accurately predicting the structural properties of phospholipid with a fully atomistic molecular model is critical for the study of pure phospholipid bilayers, mixed bilayer systems and bilayers containing proteins. The general amber force field (GAFF) has traditionally required the presence of a surface tension parameter to correctly model phospholipid bilayer properties such as area per lipid and order parameters. In this work, the GAFF partial charges for 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphate (POPA), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphochiline (POPC) and 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG) were re-parameterised utilising high-level ab initio calculations and the restrained electrostatic potential method. Simulations of pure POPA, POPC and POPG bilayers using the charge-modified GAFF and no applied surface tension are compared with available experimental data, the original GAFF model and the recent Lipid14 variant. The results indicate a significant improvement in the accuracy of the lipid model for reproducing experimental observables without the need for a surface tension parameter. The successful application of modifying the lipid charge distributions represents an alternative to the use of a surface tension parameter within GAFF, and highlights the importance of the partial charge calculations when modelling lipid bilayers.     

Optimizing oriented planar-supported lipid samples for solid-state protein NMR

Sample orientation relative to the static magnetic field of an NMR spectrometer allows study of membrane proteins in the lipid bilayer setting. The straightforward preparation and handling of extremely thin mica substrates with consistent surface properties has prompted us to examine oriented phospholipid bilayer and hexagonal phases on mica. The spectral characteristics of oriented lipid samples formed on mica are as good as or better than those on glass. Nine solvents with varying dielectric constants were used to cast lipid films or for vesicle spreading; film characteristics were then compared, and static solid-state 31P-NMR was used to characterize the degree of orientation of the hydrated lipid species. Lipids with four headgroup chemistries were tested: 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC), 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphoglycerol (POPG), 1,2-dioleoyl-sn-glycero-3-phosphate (DOPA), and 1,2-dioleoyl-sn-glycero-3-phosphoethanolamine (DOPE). Solvent affected orientation of POPG, DOPA, and DOPE, but not POPC. Film characteristics varied with solvent, with ramifications for producing homogeneous oriented lipid samples. POPC was used to optimize the amount of lipid per substrate and compare hydration methods. POPG did not orient reproducibly, whereas POPG-POPC mixtures did. DOPA showed 1-2 oriented states depending upon hydration level and deposition method. DOPE formed an oriented hexagonal phase that underwent a reversible temperature-induced phase transition to the oriented bilayer phase.